Amplifying devices, and in particular solid state amplifying devices such as bipolar transistors and field effect transistors (FETs), are undesirably subject to changes in gain due to changes in environmental temperature. As an example, for gallium arsenide (GaAs) FETs peak drift velocity and carrier drift mobility in the material decrease with increasing temperature while the pinch off voltage increases with increasing temperature. As a result, an amplifier using GaAs FETs will undesirably increase in power gain as the temperature is decreased and decrease in power gain as the temperature is increased.
A number of temperature compensation techniques are known. U.S. Pat. No. 2,951,208, issued Aug. 30, 1960, to L. E. Barton, discloses a temperature sensitive diode in the bias network of a bipolar transistor amplifier circuit. A diode suitable for use with a bipolar transistor is inexpensive and for most applications provides adequate compensation. However, a diode suitable for use with a GaAs FET is relatively expensive.
It is also known to use a temperature sensitive resistance element having a negative coefficient of temperature, such as a thermistor, in a bias circuit. See for example, U.S. Pat. No. 3,264,571, issued Aug. 2, 1966 to J. D. Meindl, et al. Such an arrangement although inexpensive, does not allow for accurate temperature compensation over a wide temperature range.